Subject: Re: [PATCH net v3] virtio_net: Do not send RSS key if it is not supported

From: "Michael S. Tsirkin" <[email-masked]>
Date: Sun, 31 Mar 2024 16:20:30 -0400

On Fri, Mar 29, 2024 at 10:16:41AM -0700, Breno Leitao wrote:
> There is a bug when setting the RSS options in virtio_net that can break
> the whole machine, getting the kernel into an infinite loop.
> 
> Running the following command in any QEMU virtual machine with virtionet
> will reproduce this problem:
> 
>     # ethtool -X eth0  hfunc toeplitz
> 
> This is how the problem happens:
> 
> 1) ethtool_set_rxfh() calls virtnet_set_rxfh()
> 
> 2) virtnet_set_rxfh() calls virtnet_commit_rss_command()
> 
> 3) virtnet_commit_rss_command() populates 4 entries for the rss
> scatter-gather
> 
> 4) Since the command above does not have a key, then the last
> scatter-gatter entry will be zeroed, since rss_key_size == 0.
> sg_buf_size = vi->rss_key_size;
> 
> 5) This buffer is passed to qemu, but qemu is not happy with a buffer
> with zero length, and do the following in virtqueue_map_desc() (QEMU
> function):
> 
>   if (!sz) {
>       virtio_error(vdev, "virtio: zero sized buffers are not allowed");
> 
> 6) virtio_error() (also QEMU function) set the device as broken
> 
>     vdev->broken = true;
> 
> 7) Qemu bails out, and do not repond this crazy kernel.
> 
> 8) The kernel is waiting for the response to come back (function
> virtnet_send_command())
> 
> 9) The kernel is waiting doing the following :
> 
>       while (!virtqueue_get_buf(vi->cvq, &tmp) &&
> 	     !virtqueue_is_broken(vi->cvq))
> 	      cpu_relax();
> 
> 10) None of the following functions above is true, thus, the kernel
> loops here forever. Keeping in mind that virtqueue_is_broken() does
> not look at the qemu `vdev->broken`, so, it never realizes that the
> vitio is broken at QEMU side.
> 
> Fix it by not sending RSS commands if the feature is not available in
> the device.
> 
> Fixes: c7114b1249fa ("drivers/net/virtio_net: Added basic RSS support.")
> Cc: [email-masked]

net has its own stable process, don't CC stable on net patches.

> Cc: [email-masked]
> Signed-off-by: Breno Leitao <[email-masked]>
> ---
> Changelog:
> 
> V2:
>   * Moved from creating a valid packet, by rejecting the request
>     completely
> V3:
>   * Got some good feedback from and Xuan Zhuo and Heng Qi, and reworked
>     the rejection path.
> 
> ---
>  drivers/net/virtio_net.c | 22 ++++++++++++++++++----
>  1 file changed, 18 insertions(+), 4 deletions(-)
> 
> diff --git a/drivers/net/virtio_net.c b/drivers/net/virtio_net.c
> index c22d1118a133..c4a21ec51adf 100644
> --- a/drivers/net/virtio_net.c
> +++ b/drivers/net/virtio_net.c
> @@ -3807,6 +3807,7 @@ static int virtnet_set_rxfh(struct net_device *dev,
>  			    struct netlink_ext_ack *extack)
>  {
>  	struct virtnet_info *vi = netdev_priv(dev);
> +	bool update = false;
>  	int i;
>  
>  	if (rxfh->hfunc != ETH_RSS_HASH_NO_CHANGE &&
> @@ -3814,13 +3815,24 @@ static int virtnet_set_rxfh(struct net_device *dev,
>  		return -EOPNOTSUPP;
>  
>  	if (rxfh->indir) {
> +		if (!vi->has_rss)
> +			return -EOPNOTSUPP;
> +
>  		for (i = 0; i < vi->rss_indir_table_size; ++i)
>  			vi->ctrl->rss.indirection_table[i] = rxfh->indir[i];
> +		update = true;
>  	}
> -	if (rxfh->key)
> +
> +	if (rxfh->key) {
> +		if (!vi->has_rss && !vi->has_rss_hash_report)
> +			return -EOPNOTSUPP;

What's the logic here? Is it || or &&? A comment can't hurt.

> +
>  		memcpy(vi->ctrl->rss.key, rxfh->key, vi->rss_key_size);
> +		update = true;
> +	}
>  
> -	virtnet_commit_rss_command(vi);
> +	if (update)
> +		virtnet_commit_rss_command(vi);
>  
>  	return 0;
>  }
> @@ -4729,13 +4741,15 @@ static int virtnet_probe(struct virtio_device *vdev)
>  	if (virtio_has_feature(vdev, VIRTIO_NET_F_HASH_REPORT))
>  		vi->has_rss_hash_report = true;
>  
> -	if (virtio_has_feature(vdev, VIRTIO_NET_F_RSS))
> +	if (virtio_has_feature(vdev, VIRTIO_NET_F_RSS)) {
>  		vi->has_rss = true;
>  
> -	if (vi->has_rss || vi->has_rss_hash_report) {
>  		vi->rss_indir_table_size =
>  			virtio_cread16(vdev, offsetof(struct virtio_net_config,
>  				rss_max_indirection_table_length));
> +	}
> +
> +	if (vi->has_rss || vi->has_rss_hash_report) {
>  		vi->rss_key_size =
>  			virtio_cread8(vdev, offsetof(struct virtio_net_config, rss_max_key_size));
>  
> -- 
> 2.43.0